The long-term goal of this project is a better understanding of the interaction of peripheral and central auditory processes that determine performance under conditions of stimulus uncertainty, that is, when one or more of the physical properties of the sounds cannot be predicted. Unlike many psychophysical studies which seek to minimize the influence of central processes, these experiments examine conditions in which central processes significantly influence or dominate performance. Our previous work has focused on the large effects of masker frequency uncertainty in the detection of a known signal. In the proposed work, we undertake a more general examination of the effects of stimulus uncertainty, adopting a common approach to the measurement of both masker and signal uncertainty. The absence of such an approach has hindered the development of more general models of the effects of uncertainty. We test the hypothesis that uncertainty effects may best be understood in terms of the ratio of relevant (to be processed) variability to irrelevant (to be ignored) variability, in an information-based analog to the ratio of signal-to-noise-powers that govern energy-based masking. We first develop stimuli and procedures which allow us to quantify the amount of masker uncertainty, and then use these to explore the effects of irrelevant uncertainty both alone and in combination with effects of relevant variation or effects of energy-based masking. The experiments proposed are grouped in three series which examine: 1) how best to measure uncertainty; 2) the sources of the uncertainty effects; and 3) the combination rules for the effects of irrelevant uncertainty, relevant uncertainty, and energy-based masking. Because stimulus uncertainty is inherent in everyday listening situations, these experiments will contribute to more realistic models of auditory processing that include both peripheral and central processes.